1. Field of the Invention
The present invention relates to a sheet-shaped oxygen absorber and a method for manufacturing such sheet-shaped oxygen absorber. More particularly, the present invention relates to a sheet-shaped oxygen absorber which is made by laminating plastic films or covering members in a sheet form over an oxygen-absorbing resin sheet, and which is cut into a desirable shape by ultrasonic heat sealing.
2. Description of the Prior Art
The technology for preventing a substance to be preserved, such as food and medicine, from suffering, from for example, oxidation, deterioration, discoloration, generation of molds, propagation of aerobic bacterium, and vermin damages, has consisted of the method of placing the substance to be preserved, together with an oxygen absorber, in a package which is resistant to gas permeation.
Oxygen absorbers currently on the market are normally of the type consisting of an air-permeable small bag containing a granular oxygen absorber, the so-called "small-bag-shaped oxygen absorber." However, if the small-bag-shaped oxygen absorber is placed in a package together with a substance to be preserved, the oxygen absorber will become mixed with the preserved substance. If the small-bag-shaped oxygen absorber is mixed with the preserved substance, there is a possibility that the small-bag-shaped oxygen absorber may be mistakenly cooked with the preserved substance. Moreover, when placing the small-bag-shaped oxygen absorber together with food in a gas-permeation-resistant package such as a bag, since the thickness of the small-bag-shaped oxygen absorber is not uniform, the surfaces of the package may become uneven and other problems in the wrapping or appearance of the product sometimes occur. Furthermore, if the small bag is broken by mishandling, it is possible that an oxygen-absorbing composition leaking out of the small bag may mix with the preserved substance.
On the other hand, as another form of an oxygen absorber which is different from the above-described small-bag-shaped oxygen absorber, various types of sheet-shaped oxygen absorbers have been suggested, which are made by dispersing an oxygen-absorbing composition in a thermoplastic resin and forming it into a sheet shape. For example, the official gazette of the Japanese Patent Laid-Open (Kokai) Publication No. SHO 55-44344 discloses a sheet-shaped oxygen absorber which is made by blending an oxygen-absorbing composition in a thermoplastic resin and forming the obtained mixture into a sheet shape. The official gazette of the Japanese Patent Laid-Open (Kokai) Publication No. SHO 56-26524 suggests a sheet-shaped oxygen absorber which is made by mixing an oxygen-absorbing composition in an expandable resin layer, forming the obtained mixture in a sheet form, and then making the expandable resin expand or bubble. However, each of the sheet-shaped oxygen absorbers described in the above-mentioned official gazettes is covered with air-permeable covering members on its top and bottom surfaces, but no treatment is given to the remaining portions (hereinafter referred to as the "periphery") of the oxygen absorber. Accordingly, since the periphery of these conventional sheet-shaped oxygen absorbers is exposed, there are problems such as leakage of the oxygen-absorbing composition or contamination of food due to contact with the oxygen-absorbing composition.
The applicant of this invention also discloses a film-shaped oxygen absorber in the official gazette of the Japanese Patent Laid-Open (Kokai) Publication No. HEI 2-72851 and a sheet-shaped oxygen absorber in the official gazette of the Japanese Patent Laid-Open (Kokai) Publication No. HEI 2-86758. These official gazettes respectively disclose a sheet-shaped oxygen absorber in which all surfaces are covered or wrapped, and which is made by laminating covering members, larger than the oxygen-absorbing sheet, on both surfaces of the oxygen-absorbing sheet which comprises an oxygen-absorbing composition contained in a thermoplastic resin, and by sealing the overlapped covering members along the periphery by heat so that the resin containing the oxygen-absorbing composition will not be exposed at the periphery.
However, in order to obtain the above-described sheet-shaped oxygen absorber which is entirely covered or wrapped, it is necessary in the manufacturing process to cut the oxygen absorber and the covering members respectively in different sizes and in desirable shapes and to laminate and seal them together. In this case, there is the problem that the shape of the oxygen absorber cannot be freely selected and there is another problem in productivity. Moreover, since the covering members are required to have a larger area, there is also a cost problem.
In the aforementioned official gazette of the Japanese Patent Laid-Open (Kokai) Publication No. HEI 2-86758, it is described that the periphery of the sheet-shaped oxygen absorber can be covered by melting and cutting the laminated sheet made of the oxygen absorber sheet and the covering members. However, although a normal heat melting and cutting method realizes less leakage of the oxygen-absorbing composition compared to the completely exposed periphery of the oxygen absorber resin sheet which is cut by a normal cutting method, it is difficult to completely cover the periphery and the leakage problem still exists. Moreover, in the normal heat melting and cutting method, heat is applied from the outside of the film, resulting in the problem of flashes remaining at the sectioned faces. Accordingly, the heat melting and cutting method has not always been satisfactory.